Clarify credentials, sessions and profiles

[lox]

Closes #137. Relates to #160.

General cleanup of profile / config parsing code. Implements the list command with tabular output:

Profile                   Credentials              Sessions
=======                   ===========              ========
ljd                       ljd                      61343032633337323134, 32663735326133373631, 61633665646639303539
ljd-admin                 ljd                      61343032633337323134, 32663735326133373631, 61633665646639303539
xyz                       ljd                      61343032633337323134, 32663735326133373631, 61633665646639303539

This of course begs the question, why do we have so many old sessions hanging around?

[mtibben]

So if I understand correctly - Profile is what's in config, Credentials is the ID for the credentials aws-vault stores (and uses to generate the session), and will correspond to the profile name or the source_profile if that's being used

It seems the challenge is working out the right UI to disambiguate between profiles/credentials and make users aware of this nuance, so this is definitely an improvement on what we currently have.

So because it seems this command is using profiles as it's main starting point, does this command list all profiles in config, or just profiles we have credentials stored for?

[mtibben]

Ha I just realised you asked the same question #137 (comment)

[lox]

Yeah, I decided c) all of the above. It will show profiles in your config and it will show credentials without a profile in the config. The only thing it won't do is show sessions that don't have a config entry.

[lox]

I'm going to automatically add a config entry with the add command to keep things consistent.